Installations of flexographic type comprise a screened cylinder, a plate cylinder, an impression system and a transfer system which feeds the sheets one by one between the plate cylinder and the impression system. The plate, which is mounted on the plate-cylinder, is a rubber element adapted to receive the ink which is transferred thereto by the screened cylinder and to apply it on the sheet to be printed during passage thereof in the nip zone between the plate cylinder and the impression system.
Installations of offset type comprise a plate cylinder, a so-called blanket cylinder and an impression cylinder. The blanket cylinder is coated on its periphery with a rubber element which is adapted to receive the ink transferred thereto by the plate cylinder to apply it on the sheet to be printed during passage thereof in the nip zone between the blanket cylinder and the impression cylinder.
In these two types of sheet-by-sheet printing process, there is a rubber element which is mounted on a first printing cylinder and which is applied, on the one hand, on a second ink supply cylinder and, on the other hand, via the sheet to,be printed, on an impression cylinder.
In theory, to effect transfer of the ink from the second cylinder onto the rubber element of the first impression cylinder, it is not necessary to exert a particular pressure between these two cylinders; it suffices that the ink lying on the surface of the second cylinder be to some extent touched by the outer face of the rubber element of the first printing cylinder. This is called "kiss impression".
In practice, it is necessary to exert such pressure due to the irregularities which-may exist in the thickness of the rubber element, particularly if it is question of flexographic printing plates, or due to the lack of straightness of the outer surface of one or the other of the cylinders. In fact, if such a pressure were not exerted, these different irregularities would be translated by variations in intensity of the print, going as far as forming non-printed zones.
The crushing of the rubber element, on the periphery of the first printing cylinder, provoked by the application of this pressure, avoids such shortcomings.
The same applies concerning the application of a pressure between the first printing cylinder and the impression cylinder, via the sheet to be printed.
Furthermore, to obtain optimum functioning of the print, aiming at eliminating any possible slide between the different cylinders which are applied against one another, it is usual to adapt adjustments aiming at obtaining a perfect equality between the peripheral linear speeds of the different cylinders as well as of the transfer system feeding the sheets to be printed.
However, Applicants have observed that, even when they adjust rotation of the different cylinders so as to obtain a perfect equality of the peripheral linear speeds of said cylinders, they did not obtain a perfect print. More precisely, this observation was made on examining the printing dots in flexography. In fact, being question of a theoretically circular printing dot, it is found on the printed sheet in a substantially oval form, with a really printed surface which is therefore greater than the theoretical surface, sometimes greater by 60% than this surface.
It is an object of the invention to propose a sheet-by-sheet printing process which overcomes this abnormal deformation of the printing dot, while making it possible to work with a crushing of the rubber element.